None of the research and development leading to the present invention was Federally sponsored.
This invention pertains to the general field of switching devices, and more specifically, to the field of microfabricated relays. Since the original concept of a microfabricated switching device was created by Petersen in 1979, many attempts have been made to develop switches and relays for applications of low power and high frequency. The goal of this work is to improve the cost-effectiveness and performance of switching technologies by using miniature, batch-fabricated, photolithographically-defined, moveable structures as part of a mechanical device.
Microfabricated electromechanical systems (MEMS) promise high lifetimes, low cost, small sizes, and faster speeds than switching devices manufactured by conventional means, and offer higher performance than solid-state devices. In many applications, particularly those in high performance instrumentation, automated test equipment, radar, and communication systems, switching devices with certain qualities are required or preferred. Specific values vary by application and are quantified where appropriate in the detailed description of the invention:
1) Relay rather than switch functionality, to isolate control signals from load signals
2) Low resistance Ohmic-contacts between the relay electrodes
3) Low power usage to toggle relay open/close states
4) Zero or very low power to maintain a particular relay open/close state
5) High precision, low cost manufacturing
6) High speed, high force mechanical closure of relay contacts
7) High speed, high force mechanical opening of relay contacts
8) Easily achieved control signals and operating requirements
Many switching device development efforts have been undertaken to obtain some of these advantages, but none have succeeded in attaining all. The switching device designs of prior art can be largely discussed in terms of two major categories of devices: those employing electrostatic actuating mechanisms and those employing bimorph actuating mechanisms. Each type of actuating mechanism has intrinsic qualities and advantages, as well as physical limitations preventing prior designs from obtaining every desirable quality listed above. These devices and mechanisms are described below, with the majority of prior microfabricated relay devices featuring single-throw actuation. Single throw actuation refers to the making and breaking of a single electrical contact when actuated, whereas double throw actuation refers to the breaking of one electrical contact and the making of a second contact when actuated.
Electrostatically actuated devices employ two (or more) bias electrodes across which a voltage is applied. Opposite charges are generated on the surfaces of the facing electrodes, and an electrostatic force is generated. If the bias electrodes are allowed to deflect towards each other, actuation is enabled. The switch or relay contact electrodes in an electrostatically actuated device would be mechanically coupled to these moving bias electrodes, so that the contact electrodes would mate together or separate as the voltage was applied and removed.
Electrostatic actuation intrinsically supports a number of the operating qualities described, and, as a result, is the most widely examined MEMS actuation mechanism for switches and relays. Electrostatic actuators enable Ohmic-contact relays and switches, although low resistances are difficult to achieve. They require effectively zero power to toggle states and effectively zero power to maintain states. A designer can employ microfabrication techniques to develop precise, low-cost electrostatic actuators. These actuators can provide high speeds, but high closure force is difficult to achieve, and they are not amenable to developing high opening forces. These actuators are difficult to design with low drive voltages (less than 10 V) typical of modem integrated circuits, though drive currents are typically negligible (less than 1 xcexcA).
The literature contains numerous examples of electrostatic MEMS switches and relays demonstrating low force actuation with very low power usage. Loo, et al., U.S. Pat. No. 6,046,659, describes a typical example of a single-throw, double-contact cantilever MEMS relay, employing an insulator-metal-insulator stack for stress compensation. Other cantilever MEMS devices employ different contact metals for improved performance, such as a relay by Yao, et al., U.S. Pat. No. 5,578,976, and a switch by Buck, U.S. Pat. No. 5,258,591. James, et al., U.S. Pat. No. 5,479,042, has double contact relays incorporating bumps to improve manufacturing. Zavracky, U.S. Pat. No. 5,638,946, adds a novel element for actuation, using separate fixed electrodes for biasing, after his early work in solid metal switches. The literature includes switching device work by Milanovi, et al. wherein devices are transferred from one substrate to another for improved high-frequency signal switching.
Several notable attempts have been made to improve performance at larger signal loads, typically by increasing device size and force at the expense of size, speed, and, reliability. A typical example is that of Lee, U.S. Pat. No. 6,054,659, with a copper device an order of magnitude larger and more forceful than the efforts previously noted. Komura et al. and Sato et al. have also developed millimeter-sized two-contact electrostatic MEMS relays for moderate signal loads. A device by Goodwin-Johansson, U.S. Pat. No. 6,057,520, reduces arcing under hot-switch conditions by varying the contact resistance of electrodes as the device opens and closes.
A few electrostatic MEMS switching devices have been designed to lower drive voltage requirements at the expense of device size, contact force, and, often, manufacturing disadvantages. Shen et al. and Pacheco have reduced voltage requirements by increasing bias electrode size and armature flexibility. Ichiya, et al., U.S. Pat. No. 5,544,001, incorporates novel use of stepped and sloped substrate bias electrodes for reducing drive voltage.
A few electrostatic MEMS devices have been designed with sets of bias electrodes to open the device with increased speed and force as compared to the passive restoring forces of deflected springs more typically found in MEMS devices. Hah, et al. is a typical example, combining torsional spring restoring forces with opposing bias electrodes to drive relays open. Kasano, et al., U.S. Pat. No. 5,278,368, describes a double-contact MEMS relay with drive-open electrodes as well as novel embedded electrets to reduce overall voltage requirements.
Bimorph actuators, unlike electrostatic actuators, transduce the control signals into mechanical deformation within the actuator itself. Bimorph (or, more generally, multimorph) actuators are comprised of layers demonstrating different physical responses to a particular stimulus. A thermal bimorph, for example, might have a first layer with a high coefficient of thermal expansion (above 10 ppm/xc2x0 C.) and a second layer with a low coefficient of thermal expansion (below 5 ppm/xc2x0 C.). When this bimorph is exposed to an increase in temperature, the relative expansion of the first layer is constrained by the intimate contact to the second layer, and the actuator curls in response. Devices employ this curl to perform work, and the forces generated by bimorphs can be much higher than those attainable by electrostatic actuators.
Bimorph actuation also intrinsically supports a number of the operating qualities described above, and, as a result, is the second most widely examined MEMS actuation is mechanism for switches and relays. They can be used in Ohmic-contact devices, and the high forces generated by bimorph actuators result in low contact resistances. They can be designed to actuate with low power to toggle states, though only certain types of bimorphs allow for low power state latching. Bimorph actuators can be made to provide high speeds and high closure force, and can be designed to provide similarly high opening forces and speeds. Some types of bimorph actuators can also be designed with low drive voltages and low drive currents.
Most switching devices with bimorph actuation mechanisms select piezoelectric bimorph actuators to keep power consumption low, and such devices typically demonstrate many of the desirable qualities previously listed. Few MEMS efforts have explored piezoelectric bimorphs actuators, however, due to the manufacturing difficulties associated with piezoelectric materials. Additionally, actuation of piezoelectric bimorphs typically requires complex high voltage waveforms to prevent hysteresis and degradation. Farrall, U.S. Pat. No. 4,620,123, describes a switching device featuring arrays of metalpiezoelectric-metal tri-layer actuators. Kornrumpf, U.S. Pat. No. 4,819,126 developed a series of piezoelectric bimorph actuators extending from a central anchor region for handling varying signal loads. Kornrumpf, U.S. Pat. No. 4,916,349, also designed a piezoelectric relay that latches states by changing residual polarization within the piezoelectric bimorph itself, allowing controllable zero-power passive latching. Tanaka, U.S. Pat. No. 4,403,166, developed a device consisting of opposing cantilever piezoelectric bimorphs, generating large closure force and travel. All of these devices were manufactured by conventional means, and featured all or many traditional piezoelectric material limitations.
Most microfabricated bimorph actuators employ thermal bimorphs due to the ease of manufacture and drive signal generation. Such devices typically require constant application of power to maintain an active state, and often have speed restrictions based on thermal transport phenomena. Field, et al., U.S. Pat. No. 5,467,068, discloses a general purpose thermal bimorph relay having stacked substrates with multiple novel contact structures. Norling, U.S. Pat. No. 5,463,233, has a temperature sensitive relay having multiple contact electrodes and an electrostatic bias electrode for temperature sensing, a device quite comparable to modern thermistors. Carr, U.S. Pat. No. 5,796,152, has developed a relay comprising engineered sets of opposing bimorphs, capable of passive mechanical latching at the expense of large size, speed, and power usage.
MEMS relays by Gevatter, et al., U.S. Pat. No. 5,666,258, and Schlaak, et al., U.S. Pat. Nos. 5,629,565 and 5,673,785, feature both bimorph and electrostatic actuation in which a piezoelectric bimorph actuator has integrated electrostatic electrodes to assist in the closing action. The advantage is an increase in closure force and reduction in drive voltage, at the penalty of heightened complexity and requiring simultaneous driving of both actuators for proper relay functionality.
Despite the demonstrated long-felt need and the active and wide-ranging efforts by numerous researchers and groups including those above, none of the resulting devices embody all of the desired attributes for high-performance signal switching for instrumentation, radar, and communication systems. The invention described herein is the first device to attain each of these qualities with few disadvantages and limitations in a double-throw switch configuration.
In the field of micromachined switches and relays, there are many devices which incorporate multimorph or electrostatic actuator elements. Multimorph actuators are used primarily because of their capacity to generate large forces for any given drive power, voltage, or electric current. Electrostatic actuators are used because of their capacity to use very low powers for actuation and holding switches or relays in an open or closed position. There has been a desire in the community to develop devices that incorporate large forces for reliable contacts while using low power, but no previous effort has been successful. This invention is the first attempt to achieve this goal, and does so by incorporating both high-force multimorph actuation with zero-power electrostatic latching mechanisms.
The operation of the invention allows for different stable states for the device. The first state is a passive state, which is the natural condition of the relay when no control signals are applied to the device. When an active state is desired, a drive control signal is applied to the relay actuator(s), where the mechanical limitations of the device prevent further deflection of the relay armatures. Once changed, it is desirable to hold the state for what may be an indefinite period of time in a latched state, so a latch control signal is applied to capacitive elements to attract them and hold them together with electrostatic forces. It is then possible to remove the drive control signals from the actuator, and the relay will remain latched. Removal of the latch control signal can then send the relay back to the passive state. The double-throw configuration allows for a second active state wherein a second electrical contact is made with the relay in a second closed position. An associated second latch state is also incorporated to provide low-power latching capabilities for the second closed position.
Of interest to readers unfamiliar with microfabricated devices is a brief introduction to terminology and units. The description of the drawings and detailed description of the invention to follow include precise terms that describe numbered elements of the drawings as they occur in the text. For the purposes of this provisional utility patent application, each term is considered a reserved descriptor in accordance with accepted relay industry terminology:
Milli-, m, is the standard S.I. prefix for one one-thousanth (1/1,000).
Micro-, xcexc, is the standard S.I. prefix for one one-millionth (1/1,000,000).
Nano-, n, is the standard S.I. prefix for one one-billionth (1/1,000,000,000).
Newton, N, is a standard S.I. unit of force equal to one kilogram-meter-per-second-squared.
Micron, xcexcm, or micrometer is a unit of length equal to one-one-thousandth of a millimeter.
Microfabrication is defined as a fabrication method of defining components delineated through photolithographic techniques made popular by the integrated circuit developer community.
Micromachining is defined as the action of delineating a microfabricated element that has been photolithographically defined, often performed by an etching process using acids or bases.
An actuation is defined as the action of opening or closing a relay or other switching device.
An actuator is defined as the energy conversion mechanism responsible for actuation.
An armature is defined as any element that is deflected or moved by an actuator in order to open or close a relay or other switching device.
A multimorph is defined as an actuator comprised of a combination of layers that change size when exposed to a stimulus, the size changes varying for two or more different layers.
A bimorph is defined as a multimorph with exactly two layers.
A multimorph layer is defined as any one layer of a multimorph, where each specific layer may or may not be sensitive to the drive stimulus defined for the multimorph.
A piezoelectric multimorph is defined as a multimorph actuator sensitive to electric voltage stimuli, wherein one or more layers have non-zero coefficients of piezoelectricity.
A thermal multimorph is defined as a multimorph actuator sensitive to heat or cold stimuli, wherein one or more layers have non-zero coefficients of thermal expansion.
A buckling multimorph is defined as a multimorph actuator sensitive to deflection stimuli, wherein one or more layers have non-zero stress at levels pursuant to buckling phenomena.
A fixed base is defined as a rigid, integral relay region that provides mechanical support.
A base substrate is defined as a microfabrication substrate forming one part of a fixed base.
A load signal is defined as the signal to be switched by a relay or other switching device.
A load signal line is defined as a port (input or output) for the load signal to be switched.
An armature contact element is defined as an element located on an armature that physically engages and/or disengages with other contact elements in order to form and/or break a conductive path for a load signal to progress from an input to an output load signal line.
A contact armature is defined as an armature that has attached armature contact elements.
A base substrate contact element is defined as an element located on a base substrate that physically engages and/or disengages with other contact elements in order to form and/or break a conductive path for a signal to progress from an input to an output load signal line.
A drive signal is defined as a signal that initiates the actuation of a relay or switch.
A drive signal line is defined as a line upon which is directed a drive signal. At least two drive signal lines are necessary for electric drive signals, one for the signal and one for reference.
A latch signal is defined as a signal that holds a relay or switch in an open or closed state.
A latch signal line is defined as a line upon which is directed a latch signal. At least two latch signal lines are necessary for electric latch signals, one for the signal and one for reference.
An armature electrode is defined as a conductive area attached to the armature, upon which latch signals or their references are directed.
A base substrate electrode is defined as a conductive area attached to the base substrate, upon which latch signals or their references are directed.
A latch electrode insulator is defined as an insulating region preventing electrical contact from occurring between the armature electrode and the base substrate electrode.
This invention covers switching speeds and signal loads that are generally small compared to relay industry standards. A functional distinction between xcexcA and mA, for example, is not made with regards to load signal strength for conventional relays, whereas the performance and design differences of microfabricated relays for these different load signals is significant. For purposes of this patent, the following speeds and signal loads are defined, noting that these classifications differ from those defined in relay industry standards:
Very fast switching times are defined as less than 100 nsec.
Fast switching times are defined as 100 nsec to 1 xcexcsec.
Moderate switching times are defined as 1 xcexcsec to 100 xcexcsec.
Slow switching times are defined as 100 xcexcsec to 10 msec.
Very slow switching times are defined as greater than 10 msec.
Very low signal loads are defined as less than 10 xcexcA DC current or 100 xcexcW RF power.
Low signal loads are defined as 10 xcexcA to 10 xcexcA or 10 xcexcW to 100 mW.
Moderate signal loads are defined as 10 xcexcA to 500 mA or 100 mW to 5 W.
High signal loads are defined as 500 xcexcmA to 5 A or 5 W to 50 W.
Very high signal loads are defined as greater than 5 A of DC current or 50 W of RF power.